The present invention relates to drum mix asphalt plants and to a system for controlling the composition of asphalt mix prepared in such a plant.
Continuous drum mix asphalt plants are generally supplied with aggregate from a plurality of cold feed bins containing aggregate of different grades, ranging from course to fine, and with liquid asphalt which is to be mixed with the aggregate. Each cold feed bin is generally equipped with a variable speed belt feeder which deposits aggregate from the feed bin onto a conveyor which passes underneath all of the feed bins and deposits the combined output of the feed bins into a combination dryer and drum mixer, where liquid asphalt is mixed with the aggregate.
In the past, to obtain an asphalt mix of a particular composition, the plant operator would normally select a desired total mix output in tons per hour and then manually set the speed of each variable speed belt feeder to the speed which, under optimal and unvarying conditions, would provide the proper amount of aggregate from its feed bin to form the desired proportion of the total tons per hour of aggregate. A single idler scale along the main conveyor following all of the feed bins was usually used to monitor the total amount of aggregate from all of the feed bins entering the drum mixer. If for any reason one of the belt feeders jammed or partially clogged or for any other reason provided more or less aggregate than would normally be expected for the set belt speed, the operator of the drum mix plant would only become aware of a slight change in the total amount of aggregate entering the drum mixer but had no way of knowing how the composition of the mix had changed. Only if the change were so great as to be visually noticeable was it possible to determine which feeder was responsible for the change in total output. Such a lack of control over the composition of the asphalt mix was very disadvantageous, especially in jobs where tight government specifications for the mix were to be met.
In some prior art systems, individual idler scales have been provided along the main conveyor following each cold feed bin. When individual idler scales are provided along the conveyor, the upstream feed bins deliver aggregate onto the conveyor, said aggregate becoming part of the tare weight for all of the downstream idler scales to be taken into account in determining the weight of aggregate being contributed by downstream feed bins.
It is believed that means have been provided in such prior art systems for manipulating the weights measured by the idler scales to determine roughly the weight of aggregate being delivered from each feed bin, calculating whether the correct proportion is being delivered by said feed bin, and changing the feeding rate from said feed bin by an arbitrary amount. However, such systems are believed to have lacked the capability to adjust the feeding rate of said feed bin by an amount related to the amount of error in the proportion, and further to have lacked the capability to store weight information in order to delay use of said information until the aggregate which it represents has reached downstream idler scales. Without the last-mentioned capability the tare weight added to downstream idler scales by aggregate from upstream feed bins must be taken to be the signal received simultaneously from the upstream idler scales. Thus, the proportion of aggregate contributed by downstream feed bins cannot be precisely determined with certainty since it is necessary to assume that the additional tare weight being delivered from upstream feed bins has remained constant during the time required for aggregate measured by upstream idler scales to reach downstream idler scales. This assumption will not hold true if the upstream feeding rate changes for some reason so that the aggregate weighed by the upstream idler scales does not weigh the same as the aggregate approaching the downstream idler scales.
Other attempts to control the composition of asphalt mix, such as that disclosed in U.S. Pat. No. 3,625,488, have involved complicated sampling devices which add significantly to the cost of an asphalt plant.